Two guys who frequently bring melon (watermelon, rockmelon, etc) for each other. The melon is usually nicely prepared (cut in regular sized cubes) to impress the other guy. They will usually eat the melon together and in front of other people to strengthen the melon bond. Melon brothers are usually very protective of each other.

Dude, don't you know we are melon brothers. I will fuck you up if say anything bad about my melon brother.